In applications such as spars, it is often desired to pull and maintain tension in the production casing when tying-back to the wellhead at the ocean floor. When tying-back the casing to the surface, the casing needs to be in tension due to fatigue, buckling, thermal growth, etc. To put the casing in tension requires that it be tensioned and locked at the surface wellhead.
A selected tensioning mechanism and method may be used to apply tension between the casing and the surface wellhead. When dealing with heavy casing over long distances, the casing tends to stretch. Since the exact amount of casing length stretch and the initial length are difficult to determine, it is desirable to compensate for this distance by an adjustable load bearing tensioning mechanism. In many spar applications, the tensioning mechanism has an axial stretch length of 3 meters or more. Various types of sealing mechanisms cooperate with the tensioning mechanism to seal between the upper end of the tensioned casing and the wellhead.
Prior patents relating to downhole adjustable hangers and particularly to an inner riser adjustable hanger are U.S. Pat. Nos. 5,566,761; 5,944,111; 4,519,633; 6,328,108; RE34,071; 4,938,289; 4,408,783; 4,465,134; 1,546,305; 3,721,292; 4,653,589; 2,660,248; 3,104,708; 3,581,817; 3,690,344; 3,976,139; 2,897,895; 3,011,552; 3,933,376; 4,343,495; 4,674,576; 4,280,719; 4,258,795; 3,861,463; 3,721,292; 1,696,844; 5,299,642; 4,919,454; 6,065,542; 2,228,505; 5,839,512; 4,433,725; and 4,995,464; OTC paper 4576; Adjustable Drilling Riser brochure from ABB Vetco Gray; Publication No. 20010045286; and Publication No. 20020100596.
While various types and styles of inner riser adjustable hangers have been provided to the hydrocarbon recovery industry, prior art inner riser adjustable hangers have significant disadvantages which have limited their acceptance. Those disadvantages include high costs and poor reliability of the sealing mechanism. Many adjustable hangers require rotation of either an inner string or an outer tubular, and in many applications rotation of a string or other tubular at the surface, particularly under conditions where that tubular is subject to high tension and/or rapid changes in tension, are undesirable for the well operator, and in turn may require more costly surface equipment.
The disadvantages of the prior art are overcome by the present invention, and the inner riser adjustable hanger and seal assembly is hereinafter disclosed. The invention also involves a method of maintaining a desired tension in a casing string, and thereafter running in a sealing assembly for sealing between the wellhead and the tensioned casing string.